Field of the Invention
The present invention relates to a toner and a two-component developer for developing (visualizing) electrostatic latent images (electrostatic charge images).
Description of the Related Art
Recently, electrophotographic image forming apparatuses (electrophotography apparatuses) such as copying machines and printers have been required to have developability and durable stability capable of stably output images having high image quality, and have been simultaneously required to cope with energy saving.
For the purpose of achieving high image quality and high developability, the properties required for the toner include high charge amount and sharp distribution of charge amount.
As a method for controlling the charge amount of the toner, a method controlling by using a charge control agent is known. Japanese Patent No. 3986488 describes as a charge control agent, a monoazo iron complex compound satisfactory in rise of electric charge.
The charge amount of a toner is significantly affected by the performance of the binder resin in the toner particle. Japanese Patent Application Laid-Open No. 2012-215857 describes the use of, as the binder resin for a toner particle, a polyester resin using an aliphatic polyhydric alcohol having a high hygroscopic property in a proportion of 70 mol % or more, and the use of, as the charge control agent, a pyrazolone monoazo metal compound having a low hygroscopic property. According to the description of the foregoing patent document, in this way, a toner can be obtained in which the charge amount is hardly decreased even in a high-humidity environment.
The use of these charge control agents allows toners relatively high in charge amount to be obtained.
For the purpose of achieving energy saving in image forming apparatuses such as copying machines and printers, the decrease of the fixing temperature of the toner is also effective. Accordingly, for the purpose of improving the low-temperature fixability of the toner, the improvement of the binder resin used in the toner particle has hitherto been made to proceed.
Japanese Patent No. 5132913 describes a technique to improve the low-temperature fixability by using, as a binder resin for a toner particle, a polyester resin synthesized in the presence of a compound having a functional group reacting with an acid or an alcohol and a long-chain alkyl group. In this polyester, the aforementioned compound is incorporated as a constitutional unit.
However, the charge control agents described in Japanese Patent No. 3986488 and Japanese Patent Application Laid-Open No. 2012-215857 are high in charging capability, and hence the charge amount distribution of the toner tends to be broad. When the charge amount distribution of the toner is broad, in particular, in a low-temperature low-humidity environment, the “scattering” of the toner tends to occur. The “scattering” as referred to herein means the state in which an excessively charged toner scatters into the peripheral non-image area (the area normally not to be developed with the toner) surrounding the image area (the area to be developed with the toner).
There has also been a tendency that the so-called selective development comes to be remarkable in which among the toner particle, particles having relatively small particle sizes tending to have high charge amounts are selectively used for development and particles having relatively large particle sizes are not used for development and accumulated in the developing unit.
When the charge amount distribution of the toner is broad, it comes to be difficult to faithfully reproduce the electrostatic latent image. Consequently, the uniformity of a halftone image is disturbed, the granular feeling of the toner is remarkable, and the image tends to undergo the occurrence of roughness.
Also, as described in Japanese Patent No. 5132913, by allowing polyester to have long chain alkyl groups, the charge amount of the toner tends to be decreased. Consequently, when development is performed by setting the same development contrast, the laid-on amount of the toner in the line area is larger relative to the laid-on amount of the toner in the solid black area, and the line/solid ratio is sometimes degraded (deviates from a predetermined value).